Airtight detection of wheels is an important link in wheel production. In order that wheels have traceability, steel grades are printed on rims below valve holes of the wheels passing the airtight detection, wherein the steel grades represent passing the airtight detection. In actual production, steel grade identifiers are still manually printed in some old production lines, so that the efficiency is low, the depths of the identifiers are not consistent, and the risk of missing printing also exists; and some automatic production lines realize automation, but the efficiency is low, and the period is long.